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Modified SNAP Setup for Growing Lettuce in the Summer

Modified SNAP system with twelve seedling plugs arranged in each grow box.

Lettuce and Summer Temperatures

Lettuce are normally grown in areas with relatively low temperatures. High temperatures can cause them to flower and turn bitter sooner. In the tropical environment of the Philippines lettuce can be successfully grown using hydroponics. This is because even though the high temperature environment keeps their vegetative period short, hydroponics makes them grow fast in this short vegetative window making them reach marketable size before they switch into their flowering stage and turn bitter. However, the summer months can be very punishing and in many areas SNAP Hydroponics growers must use their creativity to beat the summer heat.

Summertime 5-day weather outlook for Metro Manila from PAGASA?s home page.
Summertime 5-day weather outlook for Metro Manila

The biggest problem a hydroponics grower encounters in the summer is of course the heat. The ambient temperatures can easily go over 35°C (90°F). Although most plants can handle these temperatures, plants growing in hydroponics systems can fail because of this. The temperature of the nutrient solution can significantly affect the growth of plants in hydroponics. Higher temperatures lessens the amount of dissolved oxygen in the solution. It also affects other biological processes in the root system resulting in stunted growth and even crop loss.

Another problem during the summer is the intense sunlight. Sunlight intensity can be so high during the summer that it can easily penetrate through the styrofoam boxes and allow algae to grow on the nutrient solution. This is an issue because algae uses up nutrients from the solution.

The Modified System

Mr. Robert Iglesia of Farm in the City in Gumaca, Quezon, shares his method of growing lettuce in the summer months using the same materials one would use in the standard SNAP Hydroponics setup.

This method uses the same seedling raising method in the SNAP manual. However, instead of using only enough growing media to fill a quarter or up to a third of the seedling plug, we fill the entire seedling plug with growing media and transfer a healthy seedling on it. The seedling plugs are then arranged in a compact manner on the bottom half of the grow box.

Lower half of a grow box with plastic lining taped in place.
Completed lower half of the grow box.

Up to twenty-eight (28) seedling plugs can fit in a standard sized (20″⨉16″) grow box. Working solution is then added onto the grow box. The growing medium on the seedling plugs will wick the working solution moistening the entirety of the seedling plugs. Keep adding the working solution until its level is about 2-3 cm high.

Filling up the seedling plugs with growing media insulates the root system better than bare roots immersed in nutrient solution. The seedling plugs arranged in this manner acts like a wick system hydroponics. This  allows the seedling plugs to draw up working solution from the lower half of the grow box which acts as a reservoir. The compact arrangement of the seedling plugs also limits the amount of light reaching the working solution further decreasing its temperature and also preventing algae from growing on it.

Plant transpiration and evaporation will lower the level of the working solution and must be replenished regularly. You may allow the working solution in the reservoir to dry up and the seedling plugs will still contain moisture. However, never allow the medium in the seedling plugs to totally dry up. Alternate adding plain water and working solution in the reservoir. This is because nutrients are left over as the plant transpires and the water from the working solution evaporates. Constantly adding working solution will increase the nutrient concentration and can cause nutrient burn or nutrient imbalance.

When the lettuce plants are bigger, limit the seedling plugs to twelve for every grow box. This will allow us to manage them easier and gives them more room to grow. Bigger plants take up more water and will require us to refill more frequently. Limiting the number of seedling plugs per box will also limit the water uptake. The amount of light reaching the nutrient solution is still limited because even though the seedling plugs are now more spaced apart the lettuce plants now have bigger leaves.

Lettuce plants in the background are smaller and arranged more tightly than the bigger lettuce plants in the foreground.
Lettuce grown in this method.

Additional Notes

This method requires additional test and experimentation as each grower has their own growing environment. Shading net is extremely helpful during the summer months when the sunlight can be intense enough to scorch lettuce leaf. Growing heat resistant lettuce varieties is also strongly recommended.

That is it! If you have any questions please feel free to leave a comment below. As always good luck and happy growing!